1. Field of the Invention
The present invention relates to a water-absorptive composite and a process for producing the same. More particularly, the present invention relates to a water-absorptive composite exhibiting excellent water absorption capacity and a high rate of water absorption, having a low content of unpolymerized monomer, and containing highly water-absorptive polymer particles stably fixed to a fibrous substrate, and a process for producing the same.
2. Background Art
Paper, pulp, nonwoven fabrics, spongy urethane resin and the like have hitherto been used as water-retaining materials in various sanitary goods, including sanitary napkins and disposable paper diapers, as well as in various materials for agriculture. The water absorption capacity of these materials is as low as about 10 to 50 times their own weight, making it necessary to use these materials in a large amount for absorbing or retaining a large amount of water. Therefore, the above materials are very bulky, and pressing of the materials with water absorbed therein, unfavorably causes water to be readily released from the materials.
To overcome the drawbacks of the above type of water-absorptive materials, a variety of highly water-absorptive polymer materials have been proposed. For example, a graft polymer of starch (for example, Japanese Patent Publication No. 46199/1978), modified cellulose (for example, Japanese Patent Laid-Open No. 80376/1975), and crosslinked water-soluble polymers (for example, Japanese Patent Publication No. 23462/1968), and self-crosslinking polymers of alkali metal salts of acrylic acid (for example, Japanese Patent Publication No. 30710/1979).
These highly water-absorptive polymer materials have solved the above problems in their own ways and have water-absorptive properties on a considerably high level. Since, however, they are, in most cases, prepared in a powder form, the use thereof for sanitary goods, such as sanitary napkins and paper diapers, necessitates for the materials to be homogeneously dispersed on a substrate, such as tissue paper, nonwoven fabric, or cotton. The polymer powder dispersed in this way, however, cannot be stably fixed to the substrate without difficulty, and, in many cases, the polymer powder dispersed becomes to be locally agglomerated. Further, the swollen gel after water absorption cannot be stably fixed to the substrate and is readily moved from the substrate. For this reason, an absorbent article, for example, a paper diaper feels coarse and stiff after urination, creating a very uncomfortable feel for a wearer. Moreover, the above method of dispersing a powdery polymer on a substrate to prepare an absorbent article involves problems such as troublesome handling of the powder and operational difficulties in carrying out homogeneous dispersion in an efficient manner.
Japanese Patent Publication No. 67712/1991 proposes, as one method for solving the above problems, a process for producing a water-absorptive composite, comprising the steps of: applying an aqueous acrylic monomer solution in a predetermined pattern onto a formed fibrous substrate; irradiating an electromagnetic radiation or a corpuscular ionizing radiation to the substrate with the acrylic monomer solution applied thereon to convert the acrylic monomer to a highly water-absorptive polymer. According to this method, a considerable improvement in homogeneous dispersion of the polymer and stable fixation of the polymer onto the substrate is attained. Since, however, the electromagnetic radiation or corpuscular ionizing radiation is used in the conversion of the monomer to a highly water-absorptive polymer, the polymer is very likely to undergo excessive self-crosslinking, unfavorably resulting in a composite having very poor properties for absorbent articles, particularly very poor water absorption capacity which is less than half of the water absorption capacity of a composite prepared by the above method of using powdery highly water-absorptive polymer.
Japanese Patent Laid-Open No. 149609/1985 proposes a process for producing a water-absorptive composite, comprising the steps of: previously impregnating a water-absorptive organic material with an aqueous acrylate monomer solution; and adding an atomized water-soluble radical polymerization initiator or water-soluble reducing agent to polymerize the acrylate monomer. According to this process, since the water-soluble polymerization initiator is added after the impregnation of the water-absorptive organic material with the acrylate monomer, the polymerization initiator, even when added in an atomized state, reaches as droplets the monomer layer, causing "uneven polymerization", that is, a difference in degree of polymerization of the acrylic acid monomer due to a difference in concentration of the polymerization initiator on the monomer layer. Therefore, it is very difficult to completely polymerize the monomer, leaving a large amount of the monomer unreacted, which is undesirable also in the light of safety problem. Further, since the water-absorptive organic material substrate is previously impregnated with the aqueous acrylate monomer solution, the aqueous monomer solution penetrates into the inside of the substrate and is polymerized there, which would reduce the diameter of capillaries as formed between the fibers. This would lead to a lowered rate of water absorption of the resultant composite and a poor swelling of the resultant polymer upon its water absorption.
Further, Japanese Patent Laid-Open No. 11675/1989 discloses a process for producing a water-absorptive composite comprising a water-absorptive acrylic polymer and a fibrous substrate, characterized by comprising the steps of: discontinuously coating a liquid comprising a mixture of a hydrophilic acrylic monomer with a hydrophilic acrylic polymer onto a fibrous substrate; and then polymerizing the liquid. In this process, however, there is a limitation on the shape and size of discontinuously formed polymer areas. In particular, it is very difficult to form fine polymer areas. This would provide a composite with insufficient water absorption properties.
Japanese Patent Publication No. 58030/1993 has proposed an absorbent article comprising a fibrous substrate and a water-absorptive polymer discontinuously adhered to the substrate, wherein at least part of the water-absorptive polymer spherically envelops the substrate. This absorbent article may overcome to some degree the drawbacks in the above mentioned conventional products.
The present inventors, however, have found that this absorbent article also has some problems. Specifically, the adhesion of the water-absorptive polymer to the substrate is unsatisfactory when the polymer is swollen upon water absorption, so that the polymer is likely to come off from the substrate. Further, despite the intended use as water-absorptive articles, the use of a hydrophilic fibrous substrate is not considered suitable. Thus, this article is taught to be prepared by spraying an aqueous monomer solution onto a substrate. Since the catalyst incorporated in the aqueous monomer solution is not of a redox type but a simple peroxide catalyst, the polymerization is slowly initiated. Therefore, when droplets of the Aqueous monomer solution reach the substrate, the viscosity thereof is still low. In order that such a low-viscosity monomer solution can successfully grow into a polymer which envelops spherically the substrate, the substrate should be non-hydrophilic so as not to cause absorption or impregnation of the aqueous low-viscosity monomer solution (as described in the above publication, the occurrence of this phenomenon will result in the formation of a "web-like" water-absorptive polymer).
An object of the present invention is to provide a water-absorptive composite comprising a fibrous substrate bearing water-absorptive polymer particles, which contains few residual monomers, exhibits excellent water absorption capacity and a high rate of water absorption and in which the highly water-absorptive polymer particles are stably fixed to the fibrous substrate.
Another object of the present invention is to provide a process for producing the above water-absorptive composite, which overcomes the above drawbacks in the prior art, enables a highly water-absorptive polymer to be fixed in any form and pattern onto any formed fibrous substrate, and can produce the water-absorptive composite in a simple manner at a low cost.